Vacuum evaporation coating apparatus including means for precleaning substrates by ion bombardment



April 21, 1970 SEELEY ET AL 3,507,248

VACUUM EVAPORATION COATING APPARATUS INCLUDING MEANS FOR PRECLEANING SUBSTRATES BY ION BOMBARDMENT Filed June 15, 1967 POWER SUPPLY INVENTORS GERARD SEELEY PAUL A TOTTA GEORGE WALD ATTORNEY United States Patent Ofiice 3,507,248 Patented Apr. 21, 1970 3,507,248 VACUUM EVAPORATION COATING APPARATUS INCLUDING MEANS FOR PRECLEANING SUB- STRATES BY ION BOMBARDMENT Gerard Seeley, Wapping'ers Falls, Paul A. Totta, Poughkeepsie, and George Wald, Spring Valley, N.Y., assignors to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Filed June 15, 1967, Ser. No. 646,244 Int. Cl. C23c 13/12 US. Cl. 118-48 Claims ABSTRACT OF THE DISCLOSURE This disclosure is directed to an improved apparatus for permitting both sputter cleaning surfaces of articles and evaporation operations to be carried out. The articles to be treated are mounted on a negatively biased support member and a pair of metal shield members having a positive or ground potential are located within the dark space on each side of the negatively biased support member. The pair of shield members serves to prevent each article from having undesired areas subjected to sputter cleaning and/or evaporation operations.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a process and apparatus for removing surface contaminants from materials and for evaporating specific material thereon and, more particularly, to a process and apparatus for removing glass and/or contaminants from selected surface areas of semiconductor substrates by ion bombardment and then to evaporate terminal metal contacts onto specific portions of the exposed surface.

Description of the prior art D.C. sputtering or cathodic sputtering refers to the removal of atoms or molecules from the surface of a material by the impact energy of gas ions which are accelerated in an electric field. This is accomplished by creating a glow discharge between an anode and a cathode with a current flow being established between these two electrodes by electron flow to the anode and positive ion flow to the cathode. The ions are created by ionization of gas molecules existing within the glow discharge region between the anode and the cathode electrodes. The ionization results from interaction of collisions of the gas particles with the electron flow from the cathode to the anode.

This removal of surface contaminants by cathodic sputtering is known in the art and is referred to as reverse sputtering since it refers to a process whereby primary interest is in material removal rather than material deposition. An example of a reverse sputtering operation for removing surface contaminants from semiconductor substrates is, for example, shown and described in the patent application entitled Ion Bombardment Cleaning, Ser. No. 502,986, inventors Fred Barson and Johann Sturm, filed Oct. 23, 1965, and assigned to the same assignee as this application.

This previously filed co-pending application describes an apparatus wherein positive gas ions are directed at a cathodic mask containing apertures so that a substrate located behind the mask can be bombarded with those ions passing through the apparatus in the mask.

In a reverse sputtering operation for cleaning surface areas of a semiconductor substrate, masks are used to permit selective ion bombardment of desired surfaces of the semiconductor structure and to prevent ion bombardment of other surface areas. One problem associated with prior art sputtering apparatus has been the formation of halos around the surface areas of the semiconductor structure that Was ion bombardment cleaned. These undesired halos were formed because of the existence of excess sputtered material in the plasma which formed about and between the finite separation between the mask and the Wafer. Furthermore, when a sputtering apparatus was used for evaporation of conductive metals to form, for example, terminal contacts to a semiconductor structure, a halo was formed because of the redeposition of previously evaporated materials that were on the parts of the apparatus. Gold and copper, for example, are easily sputtered and it was ditficult to avoid halo formation with these metals. Hence, it was a problem to both sputter clean and evaporate metal layers in one apparatus without cleaning or removing parts therefrom after each cycle of operation. Any residual contaminants left on surface areas of the semiconductor substrate as a result of nonuniform sputtering or deposition of the previously evaporated material descreased the reliability of the semiconductor devices. In one example, subsequent evaporation operations to form terminal contacts to a semiconductor device over surfaces that were not properly cleaned resulted in the undesired formation of leakage paths to the edge of each semiconductor chip. Inferior bonding because of these contaminants also resulted in undesirable electrical resistance. As a result, these mechanical and electrical weaknesses occurring from unclean surfaces caused failures in the semiconductor product and reduced the yield.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved reverse sputtering process and apparatus for the uniform and controlled removal of surface contaminants from small selected areas of an article.

It is another object of this invention to provide a combination reverse sputtering" apparatus and evaporation apparatus to, in succession, remove surface contaminants and deposit material onto specified areas of the article without requiring repeated cleaning of toolmg.

It is a still further object of this invention to provide an improved sputtering apparatus having a double mask or shield with each mask located on opposite sides of the article so as to prevent sputtering of undesired portions thereof.

It is still another object of this invention to provide a combined sputter cleaning and evaporation apparatus having means for carrying out, in succession, the sputter cleaning and evaporation operations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with one embodiment of this invention, an apparatus is described for sputter cleaning surface portions of an article and evaporating material onto the cleaned surface portions. This apparatus comprises a container and means for evacuating the container and inserting an ionizible gas therein. Article holding means, biased at one potential, are mounted in the container for holding articles for successive sputter cleaning and evaporation operations. Shield means, biased at the opposite potential from the potential of the article holding means, are located on opposite sides of the article holding means and serve to shield the article holding means from undesired sputtering and evaporation operations. The shield means are located within the dark space created during sputtering. Heating means are provided for heating the articles to a desired temperature level to permit evaporated material to be deposited on cleaned surface portions of the article. Material evaporation means are provided for evaporating material onto the cleaned surface portions of the articles of the article holding means. Several metal layers can be deposited onto the articles which are preferably semiconductor wafers. Masks are provided to expose surface portions of the wafers. With this apparatus, a low temperature sputtering operation can be carried out because of the shielding means which comprise a pair of spherical section members located on each side of the article holding means. These shields function to prevent unnecessary bombardment of the article holding means which would raise the temperature thereof and hence, cause undesired surface inversion of the semiconductor wafers located on the article holding means.

In accordance with another embodiment of this invention, a process is described for successively sputter cleaning selected surface portions of an article and evaporating material onto the cleaned surface portions of the article. The process comprises the steps of evacuating a container having at least one article therein and introducing an ionizible gas into said container. The gas is ionized and accelerated at articles to sputter clean selected surface portions of said articles. The articles are radiantly heated after sputter cleaning the selected surface portions of the articles and material is deposited onto the sputter cleaned selected surface portions of the articles.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a broken away, elevational view of the combined sputter cleaning and evaporation apparatus of this invention with parts thereof shown in cross-section; and

FIG. 2 is an enlarged sectional view showing one article mounted for both sputter cleaning and evaporation operations.

SPECIFICATION Referring to FIGURE 1, a bell jar 10 is mounted on a support 12 with an annular seal 14 serving to hermetically seal the bell jar 10 with respect to the support 12. Port or conduit 16 substantially centrally located in the support 12 is provided for evacuating the bell jar 10 to a suitable pressure in the range of 1() torr to 5 10- torr and preferably at 5x torr. Conduit 16 is connected to a suitable vacuum or diffusion pump (not shown). After the desired pressure is reached within the bell jar 10, an inert gas is introduced into the bell jar 10 through conduit 18. The inert gas, coming from an inert gas reservoir (not shown), is preferably argon although helium can also be used. Approximately 45 microns of argon is admitted through port 18 into the bell jar, and maintained therein.

Posts 20 are securely mounted in holes in the support 12 in order to support the remainder of the assembly in the bell jar 10. Preferably, three posts are spaced 120 apart in order to evenly balance the assembly. Mounted on posts 20 is a first metal spherical section member 22 having apertures 24 located therein. A flat edge portion 25 of the first metal spherical section member 22 is mounted between clamps 26 which are preferably threadedly fastened to posts 20. Slots (not shown) are provided in the flat edge portion 25 of the first spherical section member 22 in order to permit the first spherical section member 22 to be inserted between the clamps 26 located on each post 20. Hollow insulating spacers 30 (only one of which is shown) electrically isolate the first spherical section member 22 from a second metal spherical section member 32 which is mounted on the spacers 30 above the first spherical section member 22. Screws 34 fasten the first spherical section member 22 to the second spherical section member 32 by means of insulating spacers 30. The first spherical section member 22 is located less than the dark space from the second spherical section member 32 thereby preventing sputtering action from taking place between the two spherical section members.

The first spherical section member 22 is electrically connected to the post 20 (on the right hand side of FIG. 1) which is electrically connected by lead 36 to the positive or ground terminal of power supply 38. The second spherical section member 32 is electrically connected by means of lead 40 to the negative terminal of the power supply 38. The power supply 38 is a high voltage unit capable of maintaining a potential difference of several thousand volts.

Semiconductor substrate or wafer holder assemblies 42 (five of which are shown in FIG. 1) are located in the second spherical section member 32. FIG. 2 shows an enlarged view of how one of these assemblies is mounted on the second spherical section member 32.

Referring to FIG. 2, a cup-shaped flange member 44 having an aperture 46 therein is inserted on the second section member 32 at the portion thereof defining aperture 48 located in the second section member 32. Preferably, the cup-shaped member 44 is made of tantalum or molybdenum which has a low sputtering capability. Annular member 50 is mounted within the recess formed by the cup-shaped member 44 thereby serving to support pins 52 having bases 54 which are fastened to the member 50.

The aperture 46 of the cup-shaped member 44 is smaller in diameter than the aperture 24 of the first spherical section member 22. The reason for this size selection is that it was discovered during ion bombardment cleaning that more material was removed from the exterior portion or peripheral portion defined by the bottom of the cup-shaped flange member 44 than from the inner portion of the aperture 46. This halo effect would result in undesirable removal of greater thicknesses of material from the peripheral portion of a substrate if the substrate was substantially of the same diameter as the diameter of the aperture 24 of the first section member 22.

Mounted on the annular member 50 is a mask 56 which serves also to support an article, substrate, or semiconductor wafer 58. The mask 56 is preferably of molybdenum (low sputtering metal) and is provided with a pattern of apertures thereby permitting selective ion sputter cleaning and/or evaporation to be performed on the semiconductor surface portions exposed by the apertures of the mask 56 in contact with the semiconductor substrate or wafer 58. Spring contact member in the form of a centrally apertured convex metal element is in contact with the backside of the wafer 58 and serves to resiliently hold the wafer 58 against the mask 56. The spring contact member 60 has an annular flange portion 66 which is fastened to an annular metal member 64. Openings are provided in the elements 50, 56, 62, and 64 to permit the pins 52 to be inserted therethrough. The annular member 64 is suitably held in place by means of clips 66 which have a substantially U-shaped configuration and are each provided with suitably shaped slots so as to permit reduced diameter portion 68 of each pin 52 to pass into the slot and thereby permit the upper portion of each U-shaped clip 66 to abut against head 70 of each pin 52. This provides a spring loaded assembly. Heat radiation member or means 72 mounted on and connected to annular member 64 is provided as a radiating heat source for maintaining the wafer 58 at a suitable temperature for permitting the evaporation operation to be successfully carried out. Heat radiation member 72 is preferably of stainless steel and is mounted directly above and spaced from the backside portion of the 'wafer 58 thereby permitting the radiation heating of the wafer 58 through the space from the front side of the radiation member 72 through the aperture located in the spring contact member 60 to the backside of the wafer 58.

It should be evident to those skilled in the art that any desired number of substrate or wafer holding assemblies 42 can be provided in the second spherical section member 32, as may be desired. The wafer used in this support is preferably a 1%" semiconductor wafer, but it should be evident to those skilled in the art that wafers having various configurations and sizes can be used, as desired.

Spaced above the second spherical section member 32 is a third spherical section member 74 having a plurality of quartz discs 76 mounted in apertures in the third spherical section member 74 which are located directly behind the radiation members 72. Each quartz disc 76 serves to permit heat radiated from heater assembly 78 (FIG. 1) to permeate through the disc and radiantly heat the radiation member 72. The quartz discs 7 6 permit LR. energy to readily permeate therethrough. The heater assembly 78 comprises a heating coil 80 that is supported by ceramic insulating members 82 and support pins 84 from a fourth spherical section member 86. The wires of the heating coil 80 are preferably of tungsten and are suitably connected (not shown) to an A.C. source. Three support posts 88 (only two of which are shown in FIG. 1) connect flange portion 90 of the fourth spherical section member 86 to flange portion 92 of the third section member 74. The third section member 74 is mounted on a support stop 94 that is threadedly connected to support post 20. Hence, by using a bayonet-type slot in the flange 92 of the section member 74, the section member 74 is placed on the adjustable stop 94 thereby holding the fourth section member 86 in position above the third section member 74. This also permits the third section member 74 to be mounted over the second section member 32 with the distance between both of these section members being less than the dark space that is formed during glow discharge or sputtering action.

Because of the location of the third section member 74 at a distance less than the dark space from the second section member 32, no sputtering action will occur on the backside portion of the second section member 32 thereby preventing undesirable contamination and ion cleaning thereof. The third and fourth section members 74 and 86, respectively, are electrically connected together by means of metal posts 88. The third section member 74 is electrically connected to the metal stop 94 which is electrically connected tothe post and thereby connected to the positive terminal of the power supply 38. Therefore, the third and fourth section members 74 and 86, respectively, are at a positive potential with respect to the negative potential of the second section member 32. The first and third section members 22 and 74, respectively, are at the same positive potential and since they are within the dark space from the negatively biased second section member 32 they serve to effectively shield the second section member from sputtering and evaporation operations except for the regions of the second section member 32 that are behind the apertures 24 in the first section member 22. These regions are where the wafer holding assemblies 42 are located. The fourth section member 86, being spherical in configuration, is preferably of stainless steel and serves as a heat reflector for the heating coil 80. This apparatus permits rapid access to the second section member 32 thereby facilitating removal and insertion of waters.

To permit the evaporation operation to be carried out, a suitable evaporating material 96 is located in holders 98 (only one of which is shown) having a flange portion 100 supported by pins 102 and fastened thereto by screws 104. For a chromium-coppergold evaporation process Where chromium, and then copper and then gold are in succession deposited onto the wafer 58, three material holders like the one shown by reference numeral 98 are used for holding each of the three materials for the successive evaporation operations. Chromium-copper-gold limiting lands are useful in making terminal contacts for semiconductor devices. By connecting each tungsten holder 98 to a suitably controlled A.C. source 105 for heating the holder to the evaporation temperature of the material located thereon, chromium, copper and gold are evaporated, in

succession, onto the exposed portions of the substrate or wafer 58. In order to electrically isolate each holder 98 from the support 12, insulation members 106 serve to electrically isolate and support metal base member 108 on which is fastened the pins 102. During the evaporation operation, a shutter 110 is rotatably mounted by means of post 112 as indicated by designation 114. Knob 116 serves to rotate the shutter 110 into or away from the region between the evaporating material 94 and the first section member 22. In this manner, control of the successive evaporation operations is achieved using the shutter 110.

SPUTTERING AND EVAPORATION OPERATIONS In the sputtering operation or ion bombardment cleaning of the unmasked or exposed surface portions of the semiconductor wafer 58, the argon gas inserted through conduit 18 into the bell jar 10 is ionized by means of the difference in potential applied between the cathode or second member 32, which also serves as the support member for the assemblies 42, and the remaining apparatus which is at ground or at a positive potential. Hence, the positive argon ions are accelerated into the region behind the apertures 24 in the first section member 22 into contact with the exposed portions of the substrate 58. Ion bombardment cleaning occurs across the entire portion of mask 56 and a portion of the bottom of the cup-shaped member 44 as this is the target area for the ions seeking to strike the negative cathode or second section member 32. The positive shield or first section member 22 which is located less than the dark space from the cathode or secend section member 32 prevents sputtering action from taking place except through the apertures 24 of the first section member 22. After approximately fifteen minutes of ion bombardment cleaning, with the cathode at a potential in the range of 1000 to 1500 volts and preferably at 1250 volts negative potential with respect to the anode potential on the first section member 22, the sputtering operation is terminated and the argon source is closed off. Now an evaporation operation can be carried out to form the terminal metallurgy desired for the terminal openings located in an insulating layer formed on the Wafer 58. After the sputtering operation is completed in which argon ions bombard clean the exposed regions of the semiconductor wafer, the evaporation operation is carried out.

During the evaporation operation. the heating coil is energized by an A.C. source to heat each substrate or wafer and maintain it at a temperature of approximately 250 C. One additional advantage provided by this assembly is that excess evaporated metal films deposited on the first section member 22 are not resputtered during subsequent sputter-evaporation cycles. Hence, the assembly is capable of being used for about fifty or more evaporation cycles before cleaning becomes necessary. A further advantage provided by this assembly is that the evaporated films (especially gold and copper which are easily sputtered) are absent from the sputtering plasma. This is achieved by virtue of the fact that the first section member 22 is at a ground or positive potential with respect to the second section member 32. Therefore, this eliminates undesired halation containing copper and/or gold contaminants which would occur between the mask 56 and the substrate 58.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and'details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. An apparatus for sputter cleaning surface portions of an article and evaporating material onto the cleaned surface portions comprising:

an evacuation chamber;

means for evacuating said chamber;

means for thermally evaporating material from a source for deposit on said plurality of substrates, causing said material to move from said source in diverging lines and diminish in concentration uniformly;

article holding means mounted in said chamber and a plurality of holding assemblies adapted to position said plurality of articles in the path of said moving vaporized material and contoured to cut said lines at points where the concentration is substantially the same from line to line so that said articles receive substantially uniform deposits; and

means for cleaning said articles by ion bombardment,

including means for admitting an ionizable gas into said chamber, anode means, means effecting an electrical potential across said anode means and said article holding means causing a sputter discharge to be supported Within said chamber and shield means biased positively relative to said holding means located within the dark space for shielding said article holding means from undesired sputtering, said shield means includes two shields having substantially the same contour as said article holding means and positioned on opposite sides of said article holding means and electrically connected to one another, said shields being at least coextensive with the holding assemblies and having means defining apertures to expose said articles to ion bombardment and said vaporized coating material.

2. The apparatus of claim 1 and further comprising heating means for heating the articles to a desired temperature level to permit evaporated material to be deposited on cleaned surface portions of said article.

3. An apparatus in accordance with claim 2, wherein said heating means for heating the articles to a desired temperature level to permit evaporated material to be deposited on cleaned surface portions of said article comprising a heating coil and a heat reflector located behind said heating coil.

4. An apparatus in accordance with claim 3, wherein said heat reflector comprising a spherical section member,

said heating coil being suspended from said spherical section heat reflector member.

5. An apparatus in accordance With claim 1, wherein said article holding means having a spherical section configuration.

6. An apparatus in accordance with claim 1, wherein said shield means having a spherical section configuration and electrically connected together.

7. An apparatus in accordance with claim 6, wherein one of said 'pair of spherical section members having apertures therein with each aperture substantially lined up with each of the articles of said article holding means.

8. An apparatus in accordance with claim 7, wherein the other of said pair of spherical section members having radiation permeable members with each radiation permeable member lined up behind the back of each article of said article holding means.

9. An apparatus in accordance with claim 8, wherein said radiation permeable member comprising a quartz disc.

10. An apparatus in accordance with claim 1, wherein said material evaporation means comprising a material evaporation holder and energy means for heating said material evaporation holder to the evaporation temperature of material placed on said holder.

OTHER REFERENCES R. S. Mumphries: Rev. of Scientific Instruments, vol. 37, No. 12, December 1966, pp. 17345.

ROBERT K. MIHALEK, Primary Examiner U.S. Cl. X.R. 4

ll7-l06; 204-298, 312 

